We present an analysis of the early , rising light curves of 18 Type Ia supernovae ( SNe Ia ) discovered by the Palomar Transient Factory ( PTF ) and the La Silla-QUEST variability survey ( LSQ ) . We fit these early data flux using a simple power-law ( f ( t ) = \alpha \times t ^ { n } ) to determine the time of first light ( t _ { 0 } ) , and hence the rise-time ( t _ { \mathrm { rise } } ) from first light to peak luminosity , and the exponent of the power-law rise ( n ) . We find a mean uncorrected rise time of 18.98 \pm 0.54 days , with individual SN rise-times ranging from 15.98 to 24.7 days . The exponent n shows significant departures from the simple ‘ fireball model ’ of n = 2 ( or f ( t ) \propto t ^ { 2 } ) usually assumed in the literature . With a mean value of n = 2.44 \pm 0.13 , our data also show significant diversity from event to event . This deviation has implications for the distribution of ^ { \mathrm { 56 } } Ni throughout the SN ejecta , with a higher index suggesting a lesser degree of ^ { \mathrm { 56 } } Ni mixing . The range of n found also confirms that the ^ { \mathrm { 56 } } Ni distribution is not standard throughout the population of SNe Ia , in agreement with earlier work measuring such abundances through spectral modelling . We also show that the duration of the very early light curve , before the luminosity has reached half of its maximal value , does not correlate with the light curve shape or stretch used to standardise SNe Ia in cosmological applications . This has implications for the cosmological fitting of SN Ia light curves .